1. Field of the Invention
This invention relates to an image sensor capable of detecting and correcting a defective pixel without having a storage unit for storing an address of the defective pixel.
2. Description of the Related Art
In a solid-state pickup element formed on a semiconductor substrate, such as a CMOS image sensor and a CCD (Charge Coupled Device) image sensor, a defective pixel is generated in some cases due to a local crystal defect and the like on the semiconductor substrate. Since a signal level output from such defective pixel is specific to the defective pixel and independent from an amount of incident light, quality of a picked up image can undesirably be deteriorated due to the signal output from the defective signal.
In an image pickup device of video cameras and digital cameras which have heretofore been used, a defective pixel existing in a solid-state image pickup element is detected, and address data and the like relating to the defective pixel are stored in a storage unit such as a nonvolatile memory, so that a signal output from the defective pixel is corrected based on the address data and the like of the defective pixel stored in the nonvolatile memory during image pickup.
For instance, in the case of the image pickup device provided with the CMOS image sensor, a lens of the image pickup device is light-shielded in a manufacturing stage, and a signal output from each of pixels of the image sensor is compared with a signal output from a pixel around a test pixel in a state where no light is incident to the CMOS image sensor to detect whether or not a difference between values of output levels of the signals exceeds a predetermined threshold value. The pixel outputting such specific signal output level is detected as a defective pixel.
Then, address data of the defective pixel are stored in a nonvolatile memory. After completion of the defective pixel detection, the CMOS image sensor is paired with the nonvolatile memory in which the address data of the defective pixel are stored and then incorporated in the image pickup device to be shipped. In the case where a user picks up images by the use of the image pickup device, a signal corresponding to the defective pixel in video signals output from the CMOS image sensor is corrected by a signal of a pixel in the vicinity of the defective pixel based on the address data of the defective pixel of the CMOS image sensor stored in the nonvolatile memory.
However, as described above, for the purpose of the correction of defective pixel in the solid-state image pickup element, the lens of the image pickup device is light-shielded to detect the defective pixel in the state where no light is incident to the CMOS sensor or the like, and the address data and the like of the defective pixel are stored in the memory in the manufacturing stage. Therefore, there has been a problem of an increase in process steps of the manufacturing stage. Also, it is difficult to correct a defective pixel of the CMOS image sensor generated by a spaceship irradiation after the shipping of the image pickup device which is provided with the CMOS image sensor.
As a countermeasure against such problems, for example, there has been disclosed a defect correction device correcting a defective pixel by: closing a diaphragm of a lens provided in an image pickup device at the time of inputting power to the image pickup device to establish a light-shielded state; detecting a defective pixel using a image pickup output signal of a solid-state image pickup element; recording defect data based on a detection signal from the defective pixel to retain the defect data; and by using the newest defective data during image pickup (see Patent Literature 1).
However, in this defect correction device, since address data and the like of the defective pixel are stored in a memory, it is necessary to provide a storage unit such as a memory for storing the address data of defective pixel in the image pickup device. There is another problem that the number of defective pixels to be corrected depends on memory capacity of the storage unit such as a nonvolatile memory storing the address data of defective pixels.
As a countermeasure against such problems, an automatic defect detection and correction device of a solid-state image pickup element, which detects and corrects a defective pixel generated after shipment without the provision of a storage unit such as a memory and an image pickup device using the same have been disclosed, for example (see Patent Literature 2).
[Patent Literature 1] JP-A-6-6685
[Patent Literature 2] JP-A-2002-223391